Hitherto, inorganic glass has been used as a transparent material. Inorganic glass is superior in transparency, low in water absorption, and very stable. Therefore, it has widely been used as a general-purpose optical material. There has been, however, a problem that it is very heavy as specific gravity is 2.5.
Furthermore, in the case of using inorganic glass, a heating of 350° C. or higher is necessary even in low ones. Thus, there are problems that the forming step becomes complicated and the production cost becomes high, and the like.
To solve these problems, there is proposed an organic-inorganic hybrid vitreous material that is obtained by polycondensation of an organoalkoxysilane, makes a low-temperature forming possible, and is low in water absorption coefficient (see Patent Publication 1).    Patent Publication 1: Japanese Patent Application Publication 2005-146222